Increasing oil prices and national legislation requiring the reduction of automotive carbon dioxide emissions force tire and rubber producers to produce “fuel-efficient” and thus fuel-saving tires. One general approach to obtain fuel-efficient tires is to produce tire formulations which have reduced hysteresis loss. A major source of hysteresis in vulcanized elastomeric polymers is attributed to free polymer chain ends, i.e. the section of the elastomeric polymer chain between the last cross-link and the end of the polymer chain. This free end of the polymer does not participate in the efficient elastically recoverable process and, as a result, energy transmitted to this section of the polymer is lost. The dissipated energy leads to a pronounced hysteresis under dynamic deformation. Another source of hysteresis in vulcanized elastomeric polymers is attributed to an insufficient distribution of filler particles in the vulcanized elastomeric polymer composition. The hysteresis loss of a cross-linked elastomeric polymer composition is related to its tan δ value at 60° C. (see ISO 4664-1:2005; Rubber, Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having relatively small tan δ values at 60° C. are preferred as having lower hysteresis loss. In the final tire product, this translates into a lower rolling resistance and better fuel economy.
It is generally accepted that a lower rolling resistance tire can be made at the expense of deteriorated wet grip properties. For example, if, in a random solution styrene-butadiene rubber (random SSBR), the polystyrene unit concentration is reduced with respect to the total polybutadiene unit concentration, and the 1,2-polybutadiene unit concentration is kept constant, the SSBR glass transition temperature is reduced and, as a result, both tan δ at 60° C. and tan δ at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of the tire. Similarly, if, in a random SSBR, the 1,2-polybutadiene unit concentration is reduced with respect to the total polybutadiene unit concentration, and the polystyrene unit concentration is kept constant, the SSBR glass transition temperature is reduced and, as a result, both tan δ at 60° C. and tan δ at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of the tire. Accordingly, when assessing the rubber vulcanizate performance correctly, both the rolling resistance, related to tan δ at 60° C., and the wet grip, related to tan δ at 0° C., should be monitored along with the tire heat build-up.
Hirao et al., Prog. Polym. Sci. 30 (2005) 111-182, describe the construction of multi-functionalized polymers, star-branched polymers and dendritic branched polymers.
There is a need for polymers, including modified polymers, which can be used for further optimizing dynamic properties of vulcanizates containing silica and carbon black, including low hysteresis loss and high abrasion resistance, corresponding to a high wet grip, low rolling resistance and high abrasion resistance in tires. In addition, there is a need to further decrease the vulcanizate heat build-up during thermal exposure and under mechanical stress. These needs have been met by the following invention.